Optoelectronic alignment structures for the wafer level testing of optical and optoelectronic chips

ABSTRACT

This application describes, among others, wafer designs, testing systems and techniques for wafer-level optical testing by coupling probe light to/from the top of a wafer. A wafer level test system uses optical and electronic probes to search for and align with an optoelectronic alignment structure. The test system uses a located optoelectronic alignment structure as a reference point to locate other devices on the wafer. The system tests the operation of selected devices disposed on the wafer. The optoelectronic alignment loop is also used as an alignment reference of known performance for an adjacent device of unknown performance.

This application is a continuation of U.S. Non-Provisional Utilitypatent application Ser. No. 10/820,631 filed Apr. 7, 2004 now U.S. Pat.No. 7,184,626, which claims priority from U.S. Provisional applicationNo. 60/461,041 filed Apr. 7, 2003.

BACKGROUND

This application relates to testing of integrated optical devices,integrated optoelectronic circuits and systems.

Compact optical components, devices, and systems may be monolithicallyfabricated on various substrates to form integrated optical components,devices, and systems. Such fabrication may be achieved using variousfabrication processes to process wafers, including micro fabricationprocesses used for fabricating integrated circuits (ICs),micro-electro-mechanical systems(MEMS), and other microstructures. Theintegrated optical components, devices, and systems may bemonolithically integrated with ICs, MEMS, and other microstructures onthe same chip to provide various useful functions. Fabrication of suchintegrated devices with optical components or devices generally includeswafer-level fabrication processes and packaging after dice on a singlewafer are separated into chips by, e.g., dicing or sawing. Theindividually packaged chips are then tested.

In fabrication of integrated electronic circuits, a portion of the ICtesting can be performed at the wafer level prior to separation of thedice. This wafer level testing measures the performance or identifiesdefective components and dice. Hence based on such testing at the waferlevel, defective dice may be removed from subsequent processes includingthe packaging and testing of individual chips. Labor and costsassociated with subsequent processes for the defective dice, therefore,can be significantly reduced. Various wafer-level testing systems havebeen developed for testing ICs at the wafer level.

SUMMARY

This application describes, among others, exemplary designs of waferlayouts with dice for chips having at least one integrated opticalcomponent, and associated testing systems and techniques for testingsuch chips at the wafer level. In the described exemplaryimplementations, wafers are designed with one or more optical alignmentfeatures or structures alongside the components and devices of the chipsthat are easier to find or locate by optical means than an opticalcomponent or device to be tested. Each of the optical component ordevice, and the optical alignment structure or structures may beoptically accessed from a position above the wafer so that opticalalignment and testing can be performed at the wafer level prior toseparating the wafer into individual chips. In addition, as part of thewafer design, each optical alignment feature or structure has a known,predetermined positional relationship with one or more opticalcomponents or devices to be tested. Therefore, the optical component ordevice to be tested can be found by first optically locating a nearbyoptical alignment mark and then using the known positional relationshipbetween the optical alignment mark and the optical component or deviceto accurately locate the optical component or device. Next, opticaltesting of the optical component or device can be performed at the waferlevel.

The wafer level testing of optical components and devices can identifydefective optical components and associated host dice at the waferlevel. Therefore, defective dice may be removed from subsequentfabricating processes including the packaging of separated dice intochips and testing of individual chips. In this regard, the wafer leveloptical testing can spare the labor and costs associated with subsequentprocesses for the defective dice. In addition, certain optical testingprocedures, which are conducted during testing individual chips in otherchip fabrication and testing methods, may now be performed at the waferlevel prior to separating the wafer into individual dice. The waferlevel optical testing may be automated and systematically performed at ahigh speed. Therefore, the wafer level optical testing may be used toreduce the testing at the chip level and hence reduce the testing time,labor, and associated cost. Furthermore, the optical alignment andtesting techniques described here may be used for attaching fibers tooptical ports on wafers and chips after the proper alignment is achievedbetween an optical port on the chip and a fiber to be bonded to theoptical port.

In one implementation, a probe beam from an optical probe located abovethe surface of a wafer is directed to the surface of the wafer. Anoptical alignment structure on the wafer is used to direct a portion ofthe probe beam along a direction above the wafer. The portion of theprobe beam from the optical alignment structure is used as a guide toadjust the position of the optical probe relative to the opticalalignment structure.

Based on the above implementation, a known spatial relationship betweenthe positions of each component on the wafer relative to the opticalalignment structure may be used to align the optical probe with at leastone optical component on the wafer. After this alignment, the probe beamfrom the optical probe is used to optically test the optical component.

In another implementation, a device is described to include a waferhaving a wafer surface patterned to comprise an integrated component andan optical coupler operable to couple light incident from a device abovethe wafer to the integrated component. The device also includes at leastone optical alignment structure on the wafer surface operable to reflectincident light along a predetermined direction above the wafer surface.

System implementations for the wafer level optical testing are alsodescribed. For example, one described system includes a wafer holder, awafer positioner, an optical probe, an optical probe positioner, and asystem control. The wafer holder is used to hold a wafer comprising dicepattered to include optical devices and optical alignment marks thatreflect incident light. The wafer positioner is engaged to the waferholder to control positions and orientations of the wafer. The opticalprobe is used to deliver a probe beam to the wafer at an incidentdirection above the wafer and to receive optical reflection of the probebeam from the wafer. The optical probe positioner is engaged to theoptical probe to control the position of the optical probe. The controlsystem is used to control the wafer positioner and the optical probe inaligning the optical probe to a selected position on the wafer accordingto reflection of the probe beam from the optical alignment marks.

In another implementation, an optical probe head is provided to includean array of optical waveguides and to be positioned above a wafer. Atleast one optical waveguide in the array is used to direct a probe beamto the wafer which directs at least a portion of the probe beam back tothe array. One of the optical waveguides in the array is used to receivethe portion of the probe beam. The received light in the one of theoptical waveguides is used as at least one of (1) a guide to adjust aposition of the optical probe head relative to a position on the waferto align different waveguides to different predetermined positions onthe wafer and (2) a monitor signal to test an optical component on thewafer.

These and other implementations, their variations, applications, andassociated advantages and benefits are described in greater detail inthe attached drawings, the detailed description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wafer patterned with microstructures in dice.

FIG. 2 shows one die in FIG. 1 that has optical components or devices.

FIGS. 3A and 3B show operation of optical couplers in the terminal portsof optical components or devices in FIG. 2.

FIG. 3C illustrates light exiting a fiber probe used in FIGS. 3A and 3Bin interaction with a flat surface on a wafer where the angled facet andthe tilting of the fiber probe prevents the reflection and scattering ofthe light from entering the fiber probe.

FIG. 4 shows an example of a wafer-level testing system.

FIG. 5 shows examples of optical alignment structures in the form of dotand line optical alignment marks in a wafer.

FIG. 6A illustrates optical operation of an optical alignment structurein FIG. 5 according to one implementation.

FIG. 6B shows a Littrow grating as one exemplary implementation of analignment structure.

FIGS. 7A, 7B, 8A, 8B, 8C, 8D, and 9 show examples of optical alignmentstructures and their operations.

FIGS. 10, 11A, 11B, and 11C show an optical probe with an array offibers and optical alignment of the array of fibers using variousoptical alignment structures.

FIGS. 12A, 12B, 13, and 14 illustrate examples of alignment structureshaving spatially-varying optical properties and their operations.

FIGS. 15A, 15B, and 15C show an exemplary workflow for aligning a waferfor optical testing.

FIG. 15D illustrates operation of an exemplary search based on movingthe optical probe in a circular motion.

FIGS. 16, 17, and 18 illustrate examples for optical alignment devicesthat facilitate alignment of certain optical devices such aswavelength-sensitive devices and optical detectors.

FIG. 19 shows one example of an optical probe suitable for the system inFIG. 4 having a capacitance sensor.

FIG. 20 shows an exemplary workflow for adjusting the relative verticalspacing between the optical probe and the wafer in a lateral adjustmentduring optical testing.

DETAILED DESCRIPTION

The wafers for implementations of testing techniques and systemsdescribed in this application may be made of various solid-statematerials suitable for micro fabrication processes. Examples ofsubstrate materials include semiconductor materials (e.g., silicon orGaAs), silicon-on-insulator materials, glass materials, and others. FIG.1 shows an example of a wafer 100 that is patterned to include dice 110,120, etc. Each die may include one or more micro structures. Each microstructure may be designed to perform one or more specific functions oroperations. Examples of a microstructure include, but are not limitedto, an optical component or device, an electronic component or circuit,a magnetic component or device, a mechanical component or device, anopto-electronic component or device, and a MEMS component or device. Atleast one die on the wafer 100 includes an optical component that is tobe tested based on the wafer level optical testing described in thisapplication. All dice on the wafer may be identical in someimplementations. In other implementations, dice on the same wafer may bedifferent. For example, although at least one die has at least oneoptical component integrated in the die, one or more dice on the samewafer 100 may not include an optical component, or alternatively,include one or more different optical components.

FIG. 2 shows one example of the die 110 in FIG. 1 that has both opticaland electronic components integrated within the die area 201.Alternatively, a die may be all optical. In this particular example,integrated circuits 210 are fabricated along with the opticalcomponents. One optical component is a 2×2 optical switch 220 havingoptically coupled waveguides. A control mechanism, e.g., one of thecircuits 210, may be implemented to control the switching operation. Theunits 212 may be two optical transmitters to generate optical signals tobe switched. Alternatively, the units 212 may be two detectors toreceive optical signals.

The die 110 in FIG. 2 further includes a second optical component 230illustrated as an optical Mach-Zehnder modulator formed with opticalwaveguides. The unit 232 on one terminal of the modulator 230 may be anoptical detector to receive output of the modulator 230, oralternatively an optical transmitter to send an optical signal to bemodulated by the modulator 230. The other terminal of the modulator 230is an optical port 240 which includes an optical coupler as a verticaloptical interface.

An optical component or device on the die 110 may be optically passiveor active, and may be implemented in a variety of configurations.Examples include but are not limited to an optical switch, an opticalmodulator, an optical detector, an optical splitter, an opticalwavelength-division multiplexer or demultiplexer, an arrayed waveguidegrating, an optical amplifier, an optical transmitter, an opticalattenuator, an optical filter, an optical resonator, an opticalinterferometer, etc.

Notably, the die 110 includes optical ports 240 that are located atpredetermined positions and operate to input or output optical signals.In some implementations, an optical port may be located at or near anedge of a die on the wafer. In other implementations, an optical portmay be located away from an edge of a die. For example, FIG. 2 showsthat all of the optical ports 240 are located on or near one or moreedges of a die. In certain applications, some optical ports may belocated at or near an edge of a die and at least one of the opticalports 240 may be located away from an edge within the die. Each opticalport 240 is located at a terminal end of a waveguide or an opticaldevice on the die 110 and includes an optical coupler for coupling lightinto the waveguide or device from the top of the die 110 or couplinglight out of the waveguide or device and directing the light to the topof the dice 110. This optical coupler may be designed to couple lightincident to the coupling port from a light source, a waveguide or fiberpositioned above the die 110. The incident light may be in a directionwith a predetermined fixed incident angle with respect to the normaldirection of the wafer surface of the die 110. In one implementation,for example, the incident light may be at a direction with a fixed acuteangle with respect to the normal direction of the wafer surface, such asan angle of several degrees. In another implementation, the incidentlight may be at a direction substantially perpendicular to the wafersurface.

Conversely, each optical coupler within a port 240 may also be operatedto couple output light from the waveguide or device on the wafer as anoutput light beam towards the top of the wafer surface at apredetermined output angle with respect to the normal direction of thewafer surface. For example, the output angle may be at an acute anglewith respect to the normal direction of the wafer surface or may besubstantially perpendicular to the wafer surface.

Such optical couplers allow for an external optical unit located abovethe die 110 or the wafer 100 to optically interface with opticalwaveguides, optical components or optical devices on the die 110 or thewafer 100. The external optical unit located above the die 110 or wafer100 may be, for example, an optical waveguide or an optical fiber havingan end facet that receives light from or directs light to an opticalcoupler on the die 100. This external optical unit may include multiplefibers or waveguides for sending light to and receiving light from thedie or wafer. The multiple fibers or waveguides may form a 1- or2-dimension array. In these and other implementations of opticalcoupling through the top of the die 110 or wafer 100, the individual die110 need not be separated from the wafer 100 in order to perform anoptical test on an optical component or device on the die 110. The die110 and other dice on the same wafer 100 may be tested for their opticaloperations at the wafer level. The wafer level optical testing may beused to achieve a number of advantages, including early identificationof defects in optical components and devices and thus defective dice,automated testing at the wafer level, reduced testing costs and time,and savings in labor and time associated with packaging and testing atthe chip level.

FIGS. 3A and 3B illustrate two modes of operation of the optical couplerwithin an optical port 240 described above. In FIG. 3A, the coupler 240operates as an input coupler to a waveguide or an optical device on oneof the dice on the wafer 100. A beam incident from the top of the wafersurface is directed into the coupler 240. The coupler 240 receives theincident beam and redirects the incident beam in a directionsubstantially parallel to the wafer surface into an associated waveguideor device on the wafer 100. The coupler 240 may be implemented invarious forms. For example, a grating coupler may be used as the coupler240 to redirect the beam by optical diffraction. FIG. 3B illustrates theoperation of the coupler 240 as an output coupler where an output beamfrom the waveguide or device generally parallel to the wafer surface isredirected by the optical coupler 240 to the top of the wafer surface ina predetermined direction.

In both FIGS. 3A and 3B, a single fiber is illustrated as a fiber probeand represents one specific example for the external optical unit abovethe wafer surface that optically communicates with an optical componentor device on the wafer 100. Such a fiber may be a single-mode fiber, apolarization maintaining fiber, or a multi-mode fiber. With a properlydesigned grating coupler as the coupler 240, the fiber probe may be usedto guide and direct a probe beam from the top of the wafer 100 into thecoupler 240 and to receive output light from the coupler 240 through afiber end facet of the fiber. The fiber may have an angled end facetthat is positioned above the wafer surface. The fiber probe may directlight to the wafer at a small angle with respect to the normal directionof the wafer surface, or may receive light output from the wafer at thesmall angle, or may both direct light to the wafer and receive lightfrom the wafer along the direction with this small angle. Hence, a probebeam from a light source can be delivered through this fiber to the die110 and an output from the same optical coupler 240 may be directed backto the fiber for optical detection or diagnosis. The light used with theoptical probe may be at a suitable spectral range for the intendedapplications, such as the near infrared range including but not limitedto wavelengths at or near 850 nm, 980 nm, 1150 nm, 1310 nm, 1480 nm, and1550 nm.

The fiber shown in FIGS. 3A and 3B is one example of the externaloptical unit above the wafer surface that optically communicates with anoptical component or device on the wafer 100. This external optical unitabove the wafer surface may be implemented in various ways. As describedin later sections of this application, multiple fibers such as one ormore arrays of fibers or a bundle of fibers may be used in someimplementations to simultaneously couple multiple probe beams into a dieand to receive multiple output beams from the die. A light source or anarray of light sources such as diode lasers (e.g., VCSELs) and LEDs maybe directly placed above the wafer surface to direct probe light to thewafer surface. A single or an array of optical detectors may be placedabove the wafer surface to receive light coupled out of the wafersurface for testing. Certainly, a combination of two or more above andother optical components may be used as this external optical unit abovethe wafer surface for the wafer level testing.

In the example illustrated in FIGS. 3A and 3B, the fiber probe may beoriented to be slightly off the normal direction of the wafer surface sothat the output light from the fiber probe exiting at the angled endfacet is incident to the wafer surface at an angle to prevent specularreflection of the incident light from the wafer surface from hitting theangled end facet and entering the fiber probe. FIG. 3C illustrates asituation where the incident probe beam from the fiber probe hits a flatsurface or a location other than an optical port 240 having a gratingcoupler. In absence of the diffraction by the grating coupler, theincident probe beam is reflected into a main reflection based on thespecular reflection and possibly weak secondary reflections. Tilting thefiber probe allows the reflection to be directed away from the end facetof the fiber probe. In one implementation, for example, the fiber probewith an angled end facet may be set at about 8 degrees from the normaldirection.

This mechanism for preventing reflection of incident probe light fromreturning to the fiber probe in part facilitates the use of the fiberprobe to receive output light from an optical component or device on thewafer via a grating coupler 240 and the optical alignment of the opticalprobe using optical alignment marks, features, or structures describedin this patent application. For example, in some implementations, thegrating coupler in the optical port 240 shown in FIGS. 3A and 3B may bedesigned to direct output light to the fiber probe or an opticalalignment mark on the wafer may be designed to direct input light backto the angled fiber probe along the original input path by using adiffraction grating such as a Littrow grating. Therefore, unless thefiber probe is positioned above the wafer 100 at or near an optical port240 or an optical alignment mark, the fiber probe ideally does notreceive strong light from reflection from the wafer surface. Thisapplication provides some examples of using received light from anoptical port 240 for optical detection or alignment of the fiber probeand examples of using received reflection from an optical alignment markfor alignment of the fiber probe.

A die on the wafer 100 having an electronic component may have input andoutput electrical contact pads located at, e.g., near edges of the die.Testing of such electronic components may also be included in the waferlevel testing described here and may be performed either simultaneouslyor at a different time with the optical wafer level testing. Thelocations of the contact pads and optical couplers 240 may be designedto accommodate simultaneous optical and electronic testing in some chipsso that the optical probe and electronic probe(s) do not interfere witheach other. For example, an optical detector connected to an I/O port240 with a grating coupler is tested with an optical input and anelectrical output. As another example, an electrically controlledoptical modulator has at least one electrical input, one optical inputand an optical output. Such a modulator is tested with both electricaland optical probes.

In the examples shown in FIGS. 3A, 3B, and 3C, a fiber collimator lensmay be included in the fiber facet. In an implementation, where onefiber is used to deliver a probe beam to the wafer and a separate fiberis used to receive the reflected or output beam from wafer, such a fibercollimator lens may be used to improve the collection efficiency of thefiber for receiving light.

FIG. 4 illustrates one exemplary implementation of a wafer-level testingsystem 400 for testing wafers like the wafer 100 in FIG. 1 prior toseparation of the dice. This system 400 may also be used to test dicedwafers. The testing system 400 includes a support base that holds awafer positioner, an electronic probe positioner, and an optical probepositioner. The wafer positioner supports a wafer holder or chuckconfigured to hold a wafer under test and has positioning mechanisms toadjust and control the lateral and vertical positions, and orientationsof the wafer under test. The wafer positioner operates to move the waferto position a selected die under test at a predetermined fixed positionat which electronic and optical tests are conducted. The waferpositioner further operates to position components within the selecteddie under test at desired positions relative to the optical orelectronic probe for testing.

The electronic probe positioner is used to hold and position anelectronic probe above the wafer for testing electronic, optoelectronic,or electro-optical components in a die on the wafer. The electronicprobe may be an RF or DC probe with one or more electrically conductivetips or needles as the contact leads. The electronic probe may includeone or more electronic contact leads for supplying one or more testingelectronic signals to the die and receiving one or more outputelectronic signals from the die. An electronic signal output from thedie may be generated in response to light, e.g., an optical detector onthe die. An electronic probe card with multiple contact leads, e.g., maybe used for testing the electronic components. The electronic probepositioner may be controlled manually by an operator or automatically bya motorized control mechanism. The electronic probe positioner may beused to provide fine position adjustments to the electronic probe withina die area to ensure a proper alignment between the testing contactleads and the contact pads on the die for testing.

The optical probe positioner holds an external optical unit such as anoptical probe at an approximate position above the wafer or a diced diefor performing optical testing. As an example, the optical probe mayinclude one or more optical probe heads that deliver one or more probebeams from the top of the wafer to the wafer at a predetermined heightabove the wafer surface. The optical probe may also include one or moreoptical probe receivers to receive output light beams coming out of thewafer surface. An optical waveguide may be used as the optical probereceiver, the optical probe head, or both, e.g., the optical fiber probeillustrated in FIGS. 3A and 3B which may be held in a V-groove in asubstrate or a waveguide fabricated in a substrate.

For optical testing, the wafer positioner is used to adjust the lateraland vertical positions of the wafer to an initial position under theoptical probe. The wafer positioner may include a translationalpositioner or a combination of translational positioners to control thelateral positions and the vertical position of the wafer held on achuck. For the lateral control, for example, two one-dimensionalpositioners may be combined to provide adjustments in two orthogonallateral directions X and Y. A third one-dimensional positioner may beused to control the vertical position Z of the wafer. The waferpositioner may further include a yaw adjustment to adjust theorientation of the wafer around the vertical axis Z. The waferpositioner may be designed to have two predetermined fixed verticalpositions, a “testing position” and a lower “separate position” forcontrolling the vertical positions of a wafer. At the lower separateposition, the wafer positioner separates the wafer from the electronicprobe and the optical probe. At the testing position, the waferpositioner raises the wafer to be in contact with the electronic probeand close to the optical probe with a desired spacing in the verticaldirection so that either or both of electronic testing and opticaltesting may be conducted at this position. After the wafer positionersets the wafer at the testing position, the electronic probe positionercan be used to adjust the vertical position of the electronic probe tocontact the wafer for testing. The optical probe positioner may beadjusted to finely tune the lateral positions (X and Y) of the opticalprobe and the relative vertical spacing between the optical probe andthe wafer at a predetermined distance for the optical testing. Theoptical probe positioner may also be used to finely tune the yaw angleof the optical probe, for example when the optical probe is a lineararray of fibers.

The accuracy of many commercial wafer positioners, however, may not besufficient for properly aligning an I/O optical coupler 240 to theoptical probe for optical testing in part because optical alignment inoptical testing demands higher alignment accuracy. For example, anoptical fiber end facet in the optical probe may need to be aligned toan input or output optical coupler 240 with a lateral tolerance of aboutone micron or less in various applications. This accuracy is difficultto achieve in many commercial wafer positioners in wafer testing systemsdesigned for testing electronic circuits only. Hence, the system 400implements the optical probe positioner to finely adjust the positionand orientation of the optical probe relative to the wafer under test tooptimize the optical alignment. As an example, a PZT-driven precisionpiezo positioner with a displacement range of about 100 microns may beimplemented as part of the optical probe positioner for fine adjustmentof position, and a combination of a rotation stage for yaw orientationand a manual pitch/roll stage may be used for fine adjustment oforientation. Such an optical positioner provides up to six degrees offreedom in adjusting the position and orientation of the optical proberelative to the wafer.

The system in FIG. 4 may include two separate optical probes held by twodifferent optical probe positioners. As an example, one optical probemay include at least one fiber to send probe light to the wafer and theother optical probe may include a fiber to receive reflected probe lightor light output from an optical I/O port 240 on the wafer. Additionaloptical probes may also be added when needed. Two or more electronicprobes may also be included in the system in FIG. 4.

In FIG. 4, a tester and control system is shown in the system 400 tocontrol the system operations and tests. This system may include one ormore micro processors or computers and other electronics for carryingout various processing and control operations. For a given design of diepatterns on a wafer, a position map may be generated from the givendesign in which each component, optical, electronic, or otherwise, isregistered and represented by position coordinates. The data for thisposition map may be made available to the tester and control system by,e.g., being stored in an internal memory or disk of the system or aportable memory media or device. The wafer positioner may be controlledaccording to this map to move any die and any component within the dieto the fixed position under the optical probe for testing. Theelectronic probe positioner may also be controlled based on this map toalign the electronic contacts between the electronic probe and the dieon the wafer.

The tester and control system in FIG. 4 may include testing functionsfor testing the electronic and optical components. This part of thesystem may include one or more electronic signal sources for generatingdesired electronic probing signals, electronic interfaces for outputtingand receiving electronic signals, and testing analyzers for performingthe testing. For optical testing, this part of the system may includeone or more light sources for producing probe light to the wafer, one ormore optical receivers for receiving optical signals from the wafer andconverting into electronic signals, and optical testing analyzers forperforming optical testing. With at least one electronic probe and oneoptical probe, the system in FIG. 4 may perform optical testing,electronic testing, and optoelectronic testing on a chip or a wafermounted on the wafer positioner. The optical testing, electronictesting, and optoelectronic testing may be conducted at the wafer levelprior to dicing the wafer into chips.

Optical alignments for optical testing in the system in FIG. 4 may needan alignment accuracy higher than that for aligning electronic pads forelectronic tests. The tester and control system in FIG. 4, thus, may bedesigned to include fine alignment control mechanisms to achieve suchaccuracy not readily available from many wafer testing systems designedfor testing electronic IC chips or electronic devices, components, andcircuits. The entire optical alignment control mechanism of the systemin FIG. 4 is implemented in different parts of the system and thesedifferent parts operate in combination with one another. The followingdescribes exemplary implementations of the optical alignment mechanismsin the system 400 in connection with optical alignment structures suchas alignment marks or optical devices that are designed and fabricatedon wafers. Various alignment features and mechanisms in the system 400,including coarse optical alignment control for the wafer positioner, andfine optical alignment control for the optical probe positioner, may beused to fully explore the benefits of such optical alignment structureson the wafers as described in greater detail below. It is understoodthat various alignment features and mechanisms in the system 400 mayalso be used for optical alignments for optical testing, includingwafer-level optical testing, based on other optical alignment techniqueswithout such optical alignment structures on the wafers.

In the examples described below, an optical alignment structure on awafer may be an optically reflective or diffractive structure thatdirects the majority or all of light energy in an optical probe beamincident from above the wafer to a predetermined output direction abovethe wafer. This optical output from the optical alignment structure maybe collected at a position above the wafer by an appropriately designedoptical probe positioned above the wafer. The received light from theoptical alignment structure may be used as a guide or indicator to alignthe position of the optical probe with reference to the opticalalignment structure. The use of a single optical alignment structure onthe wafer may be sufficient to align the optical probe. In someimplementations, more than one alignment structures on the wafer may beused in combination to achieve the desired optical alignment and toimprove reliability and accuracy of the optical alignment. In someimplementations, after the optical probe is aligned with reference to atleast one optical alignment structure, the optical probe may be movedrelative to the wafer to align with a target optical device such as anoptical coupler in an optical I/O port 240 described above. Thisalignment step may be achieved according to a known relative positioningrelationship between the optical alignment structure and the targetoptical device.

FIG. 5 shows one example of a wafer 500 that is fabricated with opticalalignment marks as the optical alignment structures used in opticaltesting. A portion of the wafer 500 is illustrated to include theexemplary die 110 shown in FIG. 2, and optical alignment marks 510, 520,530, 540, 550, and 560 surrounding the four sides of the die 110. Eachalignment mark is designed to be optically reflective to reflect anincident optical beam from the top of the wafer back to the top of thewafer either in its original optical path or in a different butpredetermined direction so that the reflected light can be collected bythe optical probe positioned above the wafer 500 and analyzed foroptical alignment.

Each optical alignment mark may be designed to reflect incident lightback in a retro reflection mode or to reflect light to a predeterminedknown direction. In the latter non-retro-reflection design, a separateoptical receiver positioned above the wafer 500 may be used to receiveand collect the reflected probe beam for optical alignment. When eachoptical alignment mark is retro reflective, an optical probe can use thesame optical waveguide such as a fiber that delivers the probe beam tothe wafer 500 to collect the reflected probe beam to an optical detectorcoupled to the optical waveguide. The optical reflection of an alignmentmark is designed to be much stronger than optical reflection orscattering at other parts of the wafer 500. Under this design, thereflection from an alignment mark indicates whether the probe beam hitsan alignment mark and the strength of reflection indicates whether theoptical probe is properly aligned with reference to the opticalalignment mark.

FIG. 6A illustrates an alignment in connection with the wafer 500 inFIG. 5 where a single fiber probe 610 positioned above the wafer 500 isused to both direct a probe beam 611 and to receive a reflected probebeam 612 from the wafer 500. In this specific illustration, the fiberprobe 610 is under alignment with the optical alignment mark 510. Theoptical alignment mark 500 receives the probe beam 611 from the fiberprobe 610 and reflects the probe beam back to the fiber probe 610 as thereflected probe beam 612. The fiber probe 610 collects the reflectedprobe beam 612 and delivers it to an optical receiver used in theoptical alignment control. At a given height, the relative lateralposition between the mark 510 and the fiber probe 610 is adjusted tomaximize the received optical power of the reflected probe beam 612.

FIG. 6B shows one exemplary implementation of the optical alignmentmarks used in FIG. 5 where an optical diffraction grating structure suchas a Littrow grating may be used as the reflective alignment marks. TheLittrow grating as the alignment structure operates to produce a retroreflection under the Littrow condition of d=mλ/(2ng sin θ)where m is aninteger for the diffraction order, n_(g) is the effective index in thefiber probe, and θ is the angle of the fiber probe with respect to thenormal direction of the wafer surface. A Littrow grating may be directlyfabricated on the wafer 500.

In designing the wafer 500 with alignment marks, the positions of thealignment marks are predetermined and are known. In addition, therelative positions of each component within each die with respect to theoptical alignment marks are known. These positional relationships areincluded in the position map. Hence, the alignment system may use theknown location of at least one alignment mark to align the optical probeto that alignment mark and then use the alignment mark as the referenceto move the wafer to place a selected I/O optical port 240 for anoptical component under the optical probe.

The shape and location of an optical alignment structure for a die maybe designed to allow for unambiguous identification of a location on thedie. A single optical alignment structure may be sufficient to define areference location on the die. An optically reflective dot at a knownreference location on the wafer, for example, may be used as an opticalalignment mark.

The example shown in FIG. 5 uses a set of 2 alignment marks to achievethis unambiguous identification of a reference location on the die. Themarks 510 and 520 form one set. The mark 510 is an elongated line markthat is spaced from die 110 and is parallel to one side of the die 110.The mark 520 is a dot mark located at the corner formed by the twoadjacent sides of the die 110. The dot mark 520 is separated from but isaligned with the line mark 510. In this set, the dot mark 520 is the“defining” mark whose location is to be found by using the “referencing”line mark 510 as a guide leading to the dot mark 520 for convenience andease of alignment because a line does not uniquely define a location buta dot does and a line is easier to find than a dot.

In operation, the wafer positioner is controlled to move the wafer tothe fixed position so that a selected die is visually under the opticalprobe. With this initial position as a starting point, the wafer ismoved along one axis to search for a strong optical reflection from analignment mark until a reflection is received by the optical probe andis detected. As described previously with an example shown in FIGS. 3A,3B, and 3C, the optical probe is designed to receive onlyretro-reflected light from an alignment mark or light from an opticalport 240. A fiber probe with an angled end facet in a tilted positionrelative to the normal direction of the wafer surface may be used asillustrated in FIGS. 3A through 3C and FIG. 6A. Next, the wafer is movedaround to determine whether the mark is a dot mark or a line mark. Ifthe reflection is from a dot mark, the alignment with respect to the dotmark is finely adjusted and optimized by maximizing the reflection. Ifthe reflection is determined to be from a line mark, the wafer is movedalong the found reflection line until the reflection line ends and thestrong reflection signal drops to the value close to zero. The designwith dot 520 and line 510 represents the scheme in which the line mark510 is used as a “runway” leading the system to the dot 520. Once thedot 520 is found, the system can use the position map to move wafer fromthe known location of the dot 520 to place the selected optical deviceor component under the optical probe.

There are additional three sets of alignment marks surrounding theremaining sides of the die 110 as shown in FIG. 5. The second setincludes the dot 520 and the line mark 530, the third set includes dot550 and the line 540, and the fourth set includes dot 550 and the line560. These redundant sets of alignment marks are optional and may beused to further facilitate the optical alignment, especially when thewafer positioner has some inherent alignment inaccuracy in its movement.For example, three alignment marks may be used when the x, y, and ztranslational stages in the wafer positioner are not orthogonal to oneanother.

FIGS. 7A and 7B show two different examples of optical alignment marksfor implementing automated optical alignment. In FIG. 7A, threereflective optical alignment dot marks 720 are formed on the wafer foraligning a fiber probe 701 to a grating coupler in an optical I/O port240 connected to a waveguide 710. Depending on the specific design ofeach alignment mark, the light from the fiber probe 701 may be reflectedback to the same fiber or to a different fiber by an alignment mark. Thealignment marks or structures may be lithographically defined structureslike reflective gratings, structures defined by material depositionssuch as a Bragg reflector made of various dielectric layers, orstructures defined both lithography and by material deposition such as ametal layer patterned into small reflective structures and a set ofmetal layers patterned to form an effective angle surface. Spatialseparation may be obtained by, e.g., designing a reflective structurethat sends light into another fiber or a predetermined, known directionto be collected and measured. If an alignment mark or structure is retroreflective to send light back to the fiber probe 701, optical devicessuch as optical circulators, beam splitters, and directional couplersmay be coupled to the other end of the fiber probe to properly route theoutgoing and incoming optical signals. Interferometric techniques mayalso be used to improve the contrast of the optical measurements oflight received from the wafer.

The use of optical gratings as the reflective alignment marks orstructures may be used to make the alignment system sensitive to opticalpolarization. Hence, when a polarization maintaining (PM) fiber is usedfor delivering a probe beam to the wafer or receiving light from thewafer, the optical grating reflective alignment marks may be designed toallow for optimizing the orientation of the PM fiber based on thesensitivity to the optical polarization and to maximize the opticalcoupling.

FIG. 7B show another example where reflective optical alignment linesand dots in an arrangement different from the example shown in FIG. 5based on a combination of line and dot alignment marks. The reflectivealignment dots and lines are retro reflective and hence a single fiberprobe is used to deliver the probe light and to collect reflected light.A light source and an optical detector are optically coupled to theother end of the fiber probe to respectively produce the probe light andto measure the received reflected probe light. An optical circulator iscoupled between the other end of the fiber probe and the light sourceand the detector for routing the light from the light source into thefiber and received reflected probe light form the fiber to the detector.The output of the optical detector may be used to form a feedbackcontrol to the positioning mechanism so that high reflected power ismaintained as the fiber is controlled to move on top of an alignmentline mark. FIG. 7B further shows an optical component in the die that isconnected through waveguides to two optical I/O ports 240 with gratingcouplers. After the fiber probe is aligned with respect to an alignmentmark, it can be moved to align with one of the I/O ports 240 for opticaltesting.

FIGS. 8A, 8B, 8C, and 8D show an implementation where an alignmentalgorithm may be used to automate the alignment process based on thealignment marks with minimal user intervention. The alignment marks havelines and dots on the four sides of a wafer as illustrated (FIG. 8A).The x and y axes of the wafer positioner are aligned with the x and yaxes of the wafer. After the automated alignment process is initialized,the wafer positioner is adjusted to move the fiber probe relative to thewafer to search along one axis (e.g., the x axis) until a strongreflection is detected. FIG. 8B shows the power measurement during thisstep. Next, the fiber is brought back to a position where the reflectedpower was at or near the maximal power. Subsequently, the fiber positionis moved along the y direction where the reflected power remains at themaximal value until the power drops. FIG. 8C illustrates this situation.Due to the arrangement of the dot and line marks in this example, thisposition indicates that the fiber in between an end of a line and a dot.Under this condition, the fiber is continuously moved along thisdirection until the reflected power is back again. At this point, thefiber is approximately above a dot mark. Next, a local search isperformed to adjust the relative position between the fiber and the dotmark to find a position where the reflected power is maximized. FIG. 8Dillustrates a circular search path around the dot mark for maximizingthe reflected power. After the position for the maximum reflected poweris found, the alignment with respect to this dot mark is completed. Theabove process can be repeated to align the fiber to other dot marks onthe wafer.

FIG. 9 shows another example for the alignment marks on a wafer where analignment mark is purposely located near an I/O optical port 240 on thewafer with known positional offsets. Such an alignment mark is “local”to the corresponding optical port 240 and may be used in combinationwith the “global” alignment marks on the wafer, such as those shown inFIG. 8A. Such a “local” alignment mark or structure is useful when aparticular nearby I/O optical port 240 may not be optically responsivedue to some reason. For example, the optical device or the waveguideconnected to the optical port 240 experiences large optical losses; thewaveguide connected to that optical port 240 is damaged; that opticalport is for receiving an optical input only; or the optical deviceconnected to that port 240 has a response sensitive to the lightwavelength and happens to fail to produce a detectable response at theprobe wavelength. In such circumstances, a local realignment may beperformed by using the one or more alignment marks near that opticalport 240 that does not produce an optical output in response to theoptical probe beam to ensure that the fiber probe is positioned at theright place, i.e., aligned with that optical port 240.

If the optical port 240 that does not itself produce an optical responseto a received probe beam is connected to an optical detector or a devicethat produces an electronic signal in response to light received fromthat optical port 240, the electronic probe may be used to receive thatelectronic signal from the wafer as an output to assist the opticalalignment between the optical probe and the optical port such as finealignment in addition to the use of one or more “local” alignmentstructure near the optical port 240. The electronic output may also beused during the testing of the optical port 240 and the device thatreceives light from the optical port 240.

This use of both the electronic probe and the optical probe in alignmentand testing may be applied to alignment and testing of variousoptoelectronic devices and components on the wafer. As an example, anoptoelectronic component such as a LED or a laser diode may generatelight in response to an applied electronic signal. The generated lightmay be directed to an optical port 240 which directs the light as anoutput above the wafer through the corresponding optical couplertherein. In this situation, the electronic probe may be used to supplythe electronic signal and the optical probe may be used to receive theoutput light from the optical port 240 during the testing. In anotherexample, an optical port 240 may be connected to an optical switchthrough a waveguide and the optical switch operates in response to anelectronic control signal. Hence, both the electronic probe and theoptical probe may be used in aligning and testing of the optical switch.

An optical probe for optical testing may include multiple fibers or anarray of fibers that are designed to be respectively aligned withdifferent optical components on a wafer for optical testing. Opticalwaveguides formed in a substrate may be used as an alternate to themultiple fibers or the fiber array. This use of multiplefibers/waveguides or a fiber/waveguide array in the optical probe allowsfor simultaneous testing of multiple ports and multiple optical elementson a die or a wafer. At least one of the fibers in the optical fiberprobe may be dedicated for optical alignment with respect to an opticalalignment structure which may be an optical alignment mark or an opticaldevice for either alignment or some optical function on the wafer. Oncethe dedicated fiber for alignment is aligned with the optical alignmentstructure, all other fibers in the optical probe are also respectivelyaligned with their corresponding optical components on the wafer. Afiber array for an optical probe may a one-dimensional array or atwo-dimensional array. A fiber in the fiber array may be a single-modefiber, a polarization maintaining fiber, or a multi-mode fiber. Thearray may include one or more such fibers. As an example, a single fiberarray may include a combination of the two or all three types of fibers.

FIG. 10 shows one example of using a fiber array with two or more fibersfor alignment and optical testing. In this example, three fibers 1010,1020, and 1030 are shown to form a linear fiber array corresponding to adot-shaped alignment reflector 1001 for optical alignment, a firstoptical I/O port 240A for receiving light and a second optical I/O port240B for output light, respectively. The fiber 1010 is dedicated foralignment with the alignment dot 1001 and the fibers 1020 and 1030 arededicated for optical testing of the ports 240A and 240B. The fiberarray is adjusted relative to the wafer to maximize the reflected powerfrom the alignment mark 1001 that is collected by the fiber 1010 toachieve the optical alignment of the fibers 1020 and 1030 for opticaltesting. As illustrated, the fibers in the array are engaged to a fiberarray mount. A substrate with parallel V grooves with proper spacingsmay be used to hold and fix the fibers in position to form the fiberarray.

The optical alignment of a fiber array with respect to the correspondingoptical I/O ports, optical components, or alignment structures on thedie or wafer, however, presents a challenge in aligning all fibers. Onetechnical issue is the yaw alignment of the fiber array by adjusting therotational position of the fiber array around an axis that isperpendicular to the wafer surface. FIG. 11A shows a situation where onefiber in the fiber array 1120 is aligned with a corresponding opticalport or an alignment structure in an array 1110 of ports or alignmentstructures on the die 1100 while the other fibers are misaligned withtheir corresponding ports due to a yaw of the fiber array 1120 relativeto the array 1110 of the ports on the die 1100.

The wafer positioner may have a rough yaw adjustment to change the angleof the wafer around the vertical Z axis. This rough yaw adjustment maybe used in the initial alignment of the wafer after the wafer is placedon the wafer positioner. However, this rough yaw alignment in the waferpositioner may not be sufficient to address the above alignment issue.In recognition of this, a fine yaw optimization mechanism based on a yawadjustment in the optical probe positioner and an alignment loop on thewafer may be implemented and operated in combination to address thisalignment issue.

Referring back to FIG. 4, the optical probe positioner may have its owntranslational and orientation adjustments that are independent of theadjustments in the wafer positioner to provide fine adjustments to therelative position and rotation between the optical probe and the wafer.The fine yaw adjustment in the optical probe positioner is used here toachieve the fine yaw alignment of the fiber array in the optical probe.

FIG. 11B shows the die 1100 on a wafer that includes an opticalalignment loop 1130 having two optical I/O couplers 1131 and 1132separated from each other to define a line parallel to the array 1110 ofoptical ports or optical components on the die 1100 to be aligned withthe corresponding fiber array 1120 of an optical probe. The two couplers1131 and 1132 are spaced to correspond to two fibers in the fiber array1120. The optical alignment loop 1130 includes an optical waveguide 1133connecting the two optical I/O couplers 1131 and 1132. The relativeposition between the array 1110 of optical ports or components and thealignment loop 1130 is known and the wafer positioner may be used tomove the fiber array 1120 from its aligned position with the alignmentloop 1130 to align with the array 1110 of optical ports or componentsaccording to this known relative position without additional opticalalignment.

In operation, the fiber array 1120 is first aligned with the alignmentloop 1130 by aligning two couplers 1131 and 1132 to two correspondingfibers in the fiber array 1120. One of the two fibers is used to send aprobe beam to the alignment loop 1130 and the other fiber is used toreceive light output from the alignment loop 1130. When the fiber array1120 is perfectly aligned with the alignment loop 1130, the opticalpower coupled from the fiber array 1120 into one side of the alignmentloop 1130 is received from the other side of the alignment loop 1130 andis maximized. Any yawing of the fiber array 1120 that deviates the fiberarray 1120 from this alignment condition can cause the output power fromthe alignment loop 1130 to decrease or to disappear. Hence, the fine yawadjustment in the optical probe positioner is used to vary the yawing ofthe fiber array relative to the fixed wafer to monitor the optical powerreceived in the receiving fiber in the fiber array. This yawing isperformed until the optical power in the receiving fiber is optimized.After this yaw optimization, the fiber array may be moved to align withthe array of optical ports to perform optical tests.

Alternatively, the optical alignment loop 1130 may be formed in two ofthe optical ports within the array 1110 of ports on the die 1100. FIG.11C illustrates one example of this design. In this case, after the yawalignment, the fiber array 1120 is the aligned position and is ready foroptical testing of other ports without any further adjustment of thefiber array 1120.

The fine yaw optimization for a fiber array may also be implemented withtwo or more reflective or diffractive alignment structures that eachreceive light from a fiber in the fiber array and direct received lightback to the fiber. For example, the wafer may be fabricated with a lineof two or more optical alignment marks such as dot marks that spatiallycorrespond to two or more fibers in the fiber array, respectively.Referring to FIG. 11B, the two optical ports 1131 and 1132 for thealignment loop may be replaced by two reflective optical alignment dotmarks in implementing this technique. More than two dot marks may beused. After an initial alignment to align the fibers with the alignmentmarks to receive returned light in the fibers, the optical probepositioner may be adjusted to finely vary the yaw angle of the fiberarray relative to the wafer. The total optical power received by allreceiving fibers in the fiber array may be maximized to optimize theyawing. Next, the wafer positioner is used to change the lateralposition of the wafer to place the fiber array above the array ofoptical ports to perform optical tests.

The above line and dot alignment marks and the alignment loops exemplifythe use of one or more optical reflective or diffractive alignmentstructures with a distinctive spatial pattern to define a referenceposition. When two or more alignment structures are used in combinationfor alignment, one or more the alignment structures in the combinationmay be used as referencing structures (e.g., one or more line marks) foroptically guiding the optical probe to an optical alignment structurethat defines that reference position (e.g., a dot alignment mark) in thecombination. Any combination that produces a distinctive pattern may beused. As another example, two orthogonal reflective alignment line marksthat intercept at a crossing point may be used as a combination wherethe line marks guide the optical probe and the cross point defines thereference position.

As further examples, a single alignment structure may also be used foraligning the optical probe a defined reference position. FIGS. 12A, 13,and 14 show three different examples where an alignment structure isformed close to a target I/O port 240 on the wafer with a spatiallyvarying reflectivity to indicate the location of the port 240. Such aspatially-varying alignment structure may surround the port 240. Oncethe reflective alignment structure is located, a local, more precisespatial scan of the fiber position over the found alignment structuremay be used to precisely position the fiber to the port 240 under theguidance of the varying reflectivity from the alignment structure. Forexample, a reflectivity gradient may be built in to the alignmentstructure so that a positioning control feedback mechanism may be usedto automatically follow the gradient to the desired final position ofthe fiber relative to the port 240.

FIG. 12A shows an alignment structure surrounding a port 240 with agradually-increasing reflectivity profile as the distance to the port240 decreases. FIG. 12B further shows the reflected power from thisalignment structure as the fiber position is scanned along x and ydirections through the port 240. A dip in the reflected power indicatedthe position of the final fiber position if the port 240 does not sendout light during the alignment.

FIG. 13 shows another alignment structure surrounding a port 240 with astep-wise changing reflectivity profile as the distance to the port 240changes. Concentric reflective circles 1320 with varying opticalreflectivities may be used as such an alignment structure on a wafer.The optical reflectivities of the different circles may increase ordecrease with the radius from the center of the circles so that a changein the reflected power with position may be used to indicate position ofthe fiber over the circles. An optical coupler 240 is placed at thecenter of the concentric reflective circles and is optically connectedto a waveguide 1310 or an optical device. In operation, the waferpositioner is controlled to perform a circular or spiral scan todetermine the position of the fiber relative to the concentricreflective circles 1320. The varying reflected optical power from thecircles may be used to determine the center of the circles. Next, thewafer is moved to place the fiber above the center of the circles toalign with the coupler 240.

FIG. 14 shows yet another example of an alignment structure surroundingan I/O port 240 on the wafer. In this example, the alignment structureuses multiple layers to achieve a spatially varying reflectivity profilethat can guide the fiber to the surrounded port 240. As the distance tothe port 240 changes, the layer structure of the alignment structure andthus the resultant reflectivity change accordingly.

The use of the above and other alignment structures is in part based onthe recognition that a wafer positioner may have insufficient accuracyfor optical alignment. The wafer positioner is initially used to movefrom one die to another on the wafer. One or more optical alignmentstructures allow for an accurate identification of a “local” referencefor a die, i.e., the dot mark 520 in the above example, and uses thelocal reference to make further small movements of the wafer within thescale of a die to accurately locate the final position of a targetlocation in that die. At this location, the optical probe and theselected optical coupler 240 within the die are approximately alignedand hence an optical or electronic response caused by the optical probecan be generated.

Referring back to the system 400 in FIG. 4, the fine alignment for theoptical probe may be achieved by using the optical probe positioner. Inoperation, the wafer positioner sets the initial relative positionbetween the wafer and the optical probe and maintains the wafer at thisposition. The optical probe positioner, which was set at a fixedposition in prior alignment process, is controlled to move the opticalprobe relative to a corresponding optical I/O coupler to find anoptimized location. The movement of the optical probe is controlled bythe optical probe positioner according to a search pattern, such as aspiral or circle pattern. The optical probe positioner may include highresolution mechanical stages such as stepper motors, or piezo stages.Upon completion of this search, the optical probe is fixed at theoptimized location, such as one that yields maximum received optical orelectrical signal. At this point, the optical alignment is completed andthe optical test can be carried out at this location. After completionof the test, the system then moves the wafer to place the probe atanother location for optical tests.

FIGS. 15A, 15B, and 15C show an exemplary workflow of this alignmentprocess. As discussed above, a single alignment structure on the wafermay suffice to establish an initial x, y, and z position when the waferpositioner has positioning stages orthogonal to each other. More thanone alignment structures may also be used in this alignment process.

In finely adjusting the optical probe in a selected search pattern byusing the optical probe positioner, different search mechanisms may beused. In one implementation of a “hill climbing” algorithm, for example,the optical probe positioner may be moved through a sequence of discretepositions and measurements of the received responses are taken at eachof the discrete positions. The measurements are used to estimate thegradient of the received responses as a function of the discretepositions and to predict the next position to move the optical probe toincrease the received response.

Alternatively, the fiber probe may be driven by the optical probepositioner to spatially move in a circular pattern around a center at alow frequency sufficient for the optical detector in the optical probeto respond the change of optical power in the received light. When anoptical alignment dot mark or an optical I/O port 240 is located at thecenter of the circle, the received optical power or a correspondingelectronic response from the dot mark or the port 240 remainssubstantially at a constant when the optical probe at differentpositions of the circle. Under this condition, the search is completedand the optical probe positioner is controlled to place the opticalprobe to the center of the circle. If, however, the received opticalpower or the generated electronic response varies in amplitude with theposition of the optical probe in the circle, the dot mark or the port240 is not located at the center of the circle. At this off-the-centerposition, the output response is a sinusoidal function of time and the Xor Y position is also a sinusoidal function of time.

FIG. 15D illustrates an example of the measured Y position and thesignal power as a function of time and their relative phase. Therelative phase between the position and the power indicates thedirection along which the center of the circle deviates from the dotmark or the port 240. Based on this relative phase, the optical probepositioner may be controlled to adjust the optical probe to move thecenter of the circle. At the new position, the measurements are takenand additional adjustments may be made if needed. This process continuesuntil the received power is a constant and the center of the circularmotion coincides with the dot mark or the port 240. This algorithm mayoperate at a higher speed than the hill climbing method. Oneimplementation of this algorithm a system under a trade name of“NanoTrak” and marketed by Melles Griot, Ltd., of London. See also U.S.Pat. No. 6,555,983 assigned to Melles Griot.

Certain optical components or devices in a die may not produce asufficiently measurable response to the optical probe even though theoptical alignment may be perfect. For such an optical component, it isdifficult to determine whether the optical probe is satisfactorilyaligned to the corresponding optical coupler 240 connected to theoptical component. Examples of such optical devices include certainwavelength-sensitive optical devices such as optical interferometers,resonators, optical wavelength filters when the wavelength of the probebeam happens to be between two spectral peaks in the spectral response.An Arrayed Waveguide Grating (AWG) is also such an optical device. Inaddition, optical detectors such as photodetectors and devices thatproduce a non-optical signal in response to received light.

To address this technical issue in optical alignment, certain opticaldevices that are insensitive to wavelength or exhibit some dependence onwavelength but still can produce a measurable response may be fabricatedon the die or wafer as an optical alignment device for fine alignment.In some implementations, such an optical alignment device may be locatedclose to the target optical device as an optical alignment reference.Hence, if the coupler for the alignment device is aligned with theoptical probe, the coupler for the target device near the alignmentdevice can also be aligned with the optical probe after a known,pre-determined, short move that can be sufficiently accurate. Since themove is very short due to the closeness between the alignment device andthe target device by the design, an error in alignment caused by themove may be negligible.

In one implementation, the following process may be used to achieve thealignment based on the above alignment device. First, a coarse alignmentusing the wafer positioner is performed to align the optical probe tothe optical alignment device adjacent to the target optical device. Thisalignment step may use one or more optical alignment structures on thewafer. Second, the optical probe is finely adjusted by controlling theoptical probe positioner to optimize the optical alignment between theoptical probe and the optical alignment device. Next, a short, accuratemove to the target device is performed and the target device is presumedto be aligned with the optical probe. Optical testing on the targetdevice can then be performed.

FIG. 16 shows one example of an optical alignment device 1600 adjacentto a wavelength-sensitive optical element 220 such as a Mach-Zehnderinterferometer device. The device 1600 includes an optical splitter ortap 1620 to split a fraction of a beam coupled into the device 220 fromthe coupler 1610 into a waveguide 1630. The waveguide 1630 terminates atthe same edge of the die 110 where the coupler 1610 is located. Anoptical coupler 1640 is implemented at the terminal port of thewaveguide 1630. Hence, as long as there is an optical input to the port1610, the port 1640 will produce an optical output, or vice versa.Therefore, the optical output at the coupler 1640 may be used to alignthe optical probe at the coupler 1610. The alignment for the coupler1611 at another port of the device 220 may be achieved by moving theoptical probe aligned with the coupler 1610 to the coupler 1611 in ashort and known position offset.

FIG. 17 shows another example of an optical alignment device 1700located close a target device 230 with an I/O optical coupler 240 in die110. The alignment device 1700 a waveguide loop having a waveguide 1710connected between two terminal ports having two I/O couplers 1711 and1712 on the same edge of the die 110 and close to the coupler 240. Theoptical probe may be designed to have a linear array of three fibersspatially corresponding to the ports 240, 1711, and 1712. The fiberprobe is aligned by sending a probe beam into the waveguide loop 1700and optimizing the output to achieve the alignment with the port 240 ofthe adjacent device 230. In this design, the alignment with thealignment device 1700 and the alignment with the target device 230 arecompleted at the same time.

Another application of the optical alignment loop 1700 shown in FIG. 17is to assist alignment of the optical input to an optical detector 1820that receives light from an I/O coupler 240 through a waveguide 1810 asshown in FIG. 18. The optical detector 1820 produces an electricaloutput in response to an optical input. In absence of an electronicprobe to measure the detector output, it can be difficult to align theoptical probe to the port 240 for the optical detector 1820. Thealignment loop 1700 may be placed adjacent to the port 240 for aligningthe optical probe to the port 240. The optical probe may be designed tohave a linear array of three fibers spatially corresponding to the ports240, 1711, and 1712. The fiber probe is aligned by sending a probe beaminto the waveguide loop 1700 and optimizing the output to automaticallyalign the third fiber with the port 240 for the optical detector 1820.

In the above optical alignment with the optical probe on top of thewafer, the height of the end facet of the optical probe and local wafersurface may be set at a close distance, e.g., from about 5 microns toabout 25 microns, to ensure efficient coupling of the probe light intothe optical device under test. This close spacing between the probe andthe wafer might cause a collision of the probe to the wafer during alateral movement between the optical probe and wafer because the waferheld down on a vacuum chuck is not perfectly flat. The typicalunflatness of the wafer may be, for example, about 20 to 30 microns ormore in some commercial probe stations. This collision may damage theoptical probe and the devices on the wafer.

This collision may be avoided by controlling the vertical position ofthe optical probe relative to the wafer at a safely large distance toseparate the optical probe from the wafer when laterally moving theprobe from one location on the wafer to another. After the optical probeis placed at a desired lateral position relative to the wafer, thedistance between the optical probe and the wafer may be adjusted to thedesired small spacing for optical testing. In this regard, the twovertical positions in the wafer positioner for the “testing position”and the “separate position” for setting the vertical positions of awafer may be used. During the optical testing, the wafer positioner isset to the “testing position” to be close to the optical probe. Before alateral adjustment of the relative position between the optical probeand the wafer, the wafer positioner is set to the lower separateposition to separate the wafer from the optical probe. At this loweredseparate position, the lateral adjustment can be safely made withoutcolliding the optical probe into the wafer.

During the optical testing, the spacing between the optical probe andthe wafer is set to a close distance, e.g., from about 5 microns toabout 25 microns to achieve efficient optical coupling. It is beneficialto keep this close distance during optical testing as a constant intesting different optical components at different locations on thewafer. The optical probe positioner may be used to finely adjust andmaintain this close distance between the optical probe and the wafer forthe optical testing.

In this regard, a spacing monitor and an active feedback control may beused in the optical probe positioner to monitor the distance between theoptical probe and the wafer when aligning the optical probe and tocontrol the distance at the predetermined constant distance based on themonitored spacing. In one implementation, a spacing sensor may be usedto measure the height of the optical probe from the wafer surface. Thissensor may be a capacitance spacing sensor that is mounted at theoptical probe.

FIG. 19 shows an example of an optical probe 1900 suitable forimplementing the above capacitance sensor using the wafer-level testingsystem in FIG. 4. This particular probe 1900 is built on a fiber arrayholder 1910. A fiber array 1920 with multiple fibers with predeterminedspacings is fixed to the holder 1910. The fiber array 1920 is designedto correspond to the optical couplers for one or more optical componentsor devices in certain dice on a wafer design. As an example, some of thefibers in the array 1920 may correspond to the couplers 1611, 1610 and1640 on the die in FIG. 16 or the couplers 240, 1711, and 1712 on thedie in FIG. 17. As illustrated, a capacitance sensor 1930 may be engagedto the fiber holder 1910 near the fiber probe facets to provide ameasurement for the height above the wafer surface. Based on themeasurement from the sensor 1930, the optical probe positioner controlsits vertical adjustment to set the spacing between the optical probe andthe wafer surface.

FIG. 20 shows an exemplary workflow for adjusting the vertical spacingbetween the optical probe and the wafer based on the separate verticalcontrols in the wafer positioner and the optical probe positioner.

The above optical alignment and optical testing techniques may also beused for attaching one or more fibers to optical ports on surface awafer or a chip. The fibers may be polarization maintaining fibers,single-mode fibers, multi-mode fibers, or a combination of these andother fibers. For example, the system 400 in FIG. 4 may be used to firstalign a single fiber to an optical port on the fiber or align a fiberarray to an array of optical ports on the wafer. Next, an adhesive isapplied to an optical port. Subsequently, a corresponding aligned fiberis lowered in its position to contact the optical port and the adhesivethereon. This contact is maintained until the adhesive is properly curedso that the fiber and the optical port are bonded together. The adhesivemay be a UV curable adhesive so that UV light may be used to illuminatethe contact interface between the optical port and the fiber to cure theadhesive. As another example, the adhesive may be a thermal curableadhesive and heat may locally applied to the interface to cure theadhesive. Alternatively, the adhesive may be applied to the contactinterface between the optical port and the fiber after the fiber isbrought to contact with the optical port. Other applications of theabove optical alignment and testing techniques are also possible.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade without departing from the spirit and scope of the invention.

1. An apparatus disposed on a wafer for the wafer level alignment of anoptical probe with an optical port, comprising: a first optical port; anoptical detector; an optical waveguide coupled between the first opticalport and the optical detector, where a light beam coupled to the firstoptical port propagates through the optical waveguide to the opticaldetector, the optical detector generates an electrical output inresponse to the received light beam; an optical probe positioned abovethe wafer and an electronic probe coupled to the electrical output ofthe optical detector, where the optical probe is comprised of aplurality of optical waveguides, the electronic probe is comprised of aplurality of electrical conductors and at least one of the electricalconductors is coupled to a respective electrical conductor on the wafer,and a light beam is coupled from a first one of the plurality of opticalwaveguides of the optical probe to the first optical port.
 2. Theapparatus according to claim 1, wherein the first optical port, theoptical detector and the optical waveguide coupled between the firstoptical port and the optical detector comprises an optoelectronicalignment structure.
 3. The apparatus according to claim 2, wherein theoptoelectronic alignment structure designates a structure on the waferas a location reference point, where the optical probe is aligned withthe optoelectronic alignment structure disposed on the wafer.
 4. Theapparatus according to claim 3, wherein the lateral position of theoptical probe relative to the optoelectronic alignment structure isadjusted to optimize the optical alignment of the optical probe with theoptoelectronic alignment structure.
 5. The apparatus according to claim3, and further comprising a plurality of optical structures disposed onthe wafer, where the alignment of the optical probe to theoptoelectronic alignment structure provides an alignment reference ofknown performance, with respect to the alignment of the optical probe toat least one of the plurality of optical structures.
 6. The apparatusaccording to claim 3, and further comprising a plurality of opticalstructures disposed on the wafer, where the alignment of the opticalprobe with the optoelectronic alignment structure enables alignment withat least one of the plurality of optical structures.
 7. The apparatusaccording to claim 1, and further comprising: a second optical portdisposed on the wafer, a second light beam coupled from the opticalprobe to the second optical port, a first distance between the first andsecond optical ports, a second distance between the first and secondlight beams directed down, where the first and second distances aresubstantially equal.
 8. The apparatus according to claim 7, and furthercomprising a plurality of optical structures disposed on the wafer. 9.The apparatus according to claim 8, and further comprising a pluralityof positional relationships comprised of a positional relationshipbetween the optoelectronic alignment structure and each of the pluralityof optical structures.
 10. The apparatus according to claim 9, andfurther comprising a position map, where the plurality of positionalrelationships is stored in the position map.
 11. The apparatus accordingto claim 8, and further comprising an optical waveguide coupled betweenthe second optical port and a first optical structure of the pluralityof optical structures, where the second light beam optically coupled tothe second optical port is coupled to the first optical structure viathe optical waveguide, and the second light beam is used to test theoperation of the first optical structure.
 12. The apparatus according toclaim 11, wherein the first optical structure comprises a wavelengthsensitive optical structure.
 13. The apparatus according to claim 7,wherein the second optical port comprises a wavelength sensitive opticalstructure.
 14. The apparatus according to claim 1, wherein the first andsecond light beams directed down are at an acute angle with respect tothe normal to the surface of the wafer.
 15. The apparatus according toclaim 1, wherein the first and second light beams directed down aresubstantially parallel to the normal with respect to the surface of thewafer.
 16. The apparatus according to claim 1, wherein the optical probehas an angled end facet, where the angled end facet is substantiallyparallel to the surface of the wafer.
 17. The apparatus according toclaim 2, and further comprising: an optical probe positioner, where theoptical probe positioner controls the position of the optical probe withrespect to the wafer, an electronic probe positioner, where theelectronic probe positioner controls the position of the electronicprobe with respect to the wafer, a wafer positioner, where the waferpositioner controls the position of the wafer with respect to theoptical probe, and a control system to control the optical probepositioner, the electronic probe positioner and the wafer positioner.18. The apparatus according to claim 17, wherein the control systemlocates the optoelectronic alignment structure on the wafer.
 19. Theapparatus according to claim 17, wherein the control system aligns theposition and orientation of the optical probe with respect to theoptoelectronic alignment structure on the wafer.
 20. The apparatusaccording to claim 17, wherein the control system tests the operation ofthe optoelectronic alignment structure on the wafer.
 21. The apparatusaccording to claim 17, and further comprising a plurality of opticalstructures disposed on the wafer.
 22. The apparatus according to claim21, and further comprising a plurality of positional relationshipscomprised of a positional relationship between the optoelectronicalignment structure and each of the plurality of optical structures. 23.The apparatus according to claim 21, and further comprising a positionmap, where the plurality of positional relationships between theoptoelectronic alignment structure and the plurality of opticalstructures is stored in the position map.
 24. The apparatus according toclaim 23, wherein the control system moves the optical probe to aposition above a first one of the plurality of optical structures, byusing the position map to provide the positional coordinates of thefirst optical structure relative to the optoelectronic alignmentstructure.
 25. The apparatus according to claim 24, and furthercomprising an optical waveguide coupled between the first opticalstructure and a second optical structure, where the first opticalstructure comprises an optical port, a light beam coupled from theoptical probe to the first optical structure is optically coupled to thesecond optical structure via the optical waveguide, and the light beamis used to test the operation of the second optical structure.
 26. Theapparatus according to claim 1, wherein at least one of the plurality ofoptical waveguides in the optical probe is comprised of an opticalfiber, where the optical fiber is selected from a group comprising: asingle mode fiber, a polarization maintaining fiber, a multi-mode fiberand a lensed fiber.
 27. The apparatus according to claim 1, wherein theplurality of optical waveguides in the optical probe is comprised of afiber array comprised of a plurality of optical fibers, where at leastone of the plurality of optical fibers is selected from a groupcomprising: a single mode fiber, a polarization maintaining fiber, amulti-mode fiber and a lensed fiber.
 28. An apparatus disposed on awafer for the wafer level alignment of an optical probe with an opticalport, comprising: a first optical port; an optical detector; an opticalwaveguide coupled between the first optical port and the opticaldetector, where a light beam coupled to the first optical portpropagates through the optical waveguide to the optical detector, theoptical detector generates an electrical output in response to thereceived light beam, wherein the wafer is selected from a groupcomprising: silicon, silicon on insulator (SOI), silicon on sapphire(SOS), silicon on nothing (SON) and a first layer of monocrystallinesilicon, a second layer of dielectric material disposed on the firstlayer, a third layer of monocrystalline silicon disposed on the secondlayer, a fourth layer of dielectric material disposed on the thirdlayer, a fifth layer of monocrystalline silicon disposed on the fourthlayer.